In today's electronic age, many electronic devices (e.g., wireless phones, personal digital assistants, audio players, etc.), medical diagnostic equipment, consumer industrial appliances, automotive components, and other devices include display screens that are protected by thin plastic windows or lenses. Such light transmission devices are typically produced using injection molding or extruding processes. In most applications, it is desirable for such light transmission devices to be inexpensive, impact and scratch resistant, optically clear, and distortion-free.
An important issue in producing such light transmission devices is the management of stress in the fabrication of the device. Stress is basically force per unit area. The resulting phenomenon in the material is actually induced stress, which is the residual effect of the molding process. The more elastic the part or material, the less permanent the stress, and this relationship is the modulus of elasticity.
Stress may be induced during the primary phase of fabrication (e.g., through the molding process, including the injection mold) and also in ancillary and/or downstream processes (e.g., through introduction of chemicals and/or mechanical treatment of the device). Some stresses may be relieved after fabrication, for example, through annealing. In an injection molding process, the results of stress are mostly permanent, unless a secondary stress relief process is introduced.
Stress, particularly undefined and uncontrolled stress, is normally undesirable in any functional material. An exception to this as an example may be a “scribed” part (i.e., a part with an intentionally introduced mechanical stress that allows the part to be broken at a certain point as a function of use). Thus, one consideration in the manufacture of light transmission devices may be the mitigation of “unintentional” stress.
Polymer flow is non-Newtonian. Newtonian flow behavior means that the viscosity does not change with a change in shear rate (flow rate). Since plastic is a polymer, there is a relationship between shear rate and viscosity. In the flow dynamic, increased alignment decreases viscosity. Decreased and consistent viscosity is an indication that shear rate increases. Shear stress is the stress caused by internal layers of molecules flowing at different speeds. Nominal flow rate is the flow rate obtainable with relatively low pressure drops. Molds with varying flow rates caused by the runner geometry introduce pressure variations and/or drops that change the varying internal velocity and speed, thereby inducing stress.
One problem that can be caused by uneven molecular orientation or internal stresses in the light transmission device is birefringence. Birefringence occurs when the lens material has multiple indices of refraction. This causes beams of light to travel at different velocities, which can produce a rainbow effect when the lens is viewed at varying angles.